Skip to content


What is a heat battery and how does it work?

A heat battery is a device that enables storage of heat energy when available in excess, so that it can be used at a later point in time, when needed. The HEAT-INSYDE heat battery uses a salt hydrate that can store and release heat with high energy power. It is a thermochemical storage technology: it uses water and salt, which undergo a reversible reaction with each other. As long as the water is not in contact with the salt, the heat is stored. This ensures that thermochemical storage is a very good storage solution if heat has to be stored for a longer period of time (more than 2 days).

Why is the heat battery needed?

Renewable energy sources are in continuous development and increasing use. However, a downside is that their energy production is weather dependent. Solar energy will not be produced in sufficient quantities on a grey day and not enough wind energy will result on a windless day. On the other hand, intensely sunny or windy days may produce excess solar or wind energy that will go unused. A heat battery is a device that enables storage of energy when produced in excess so that it can be used at another point in time, when needed. Energy produced in excess can be stored in many ways, for instance as electric energy in batteries, as mechanical energy by pumping water up into a reservoir, as direct heat energy by heating up a water tank or by conversion into fuels. All storage solutions come with very different characteristics in terms of speed, efficiency and, of course, cost. Depending on the application, a different storage technology might be preferable. For supplying a household with heating and warm water, a heat battery offers a very suitable and competitive storage solution.

What are the main differences compared to a water buffer?

The most important difference is the storage period, which is infinite for our heat battery. A water buffer will always lose energy to its environment during storage because it uses the heat capacity of water. This is not the case with the thermochemical storage technology in the HEAT-INSYDE battery: it uses water and salt, which undergo a reversible reaction with each other. As long as the water is not in contact with the salt, the heat is stored. This ensures that thermochemical storage is a much better system if heat has to be stored for a longer period of time (more than 2 days).

How do you increase the performance of a heat pump with the heat battery?

We will focus on applying the heat battery for the moments when the air/water heat pump is not performing well: producing high temperatures for hot tap water or during cold winter days. The heat pump can be used when it is capable of performing well, but at moments when it is underperforming the heat battery will support the pump and take over in terms of power or temperature.

Is the material used to storage heat in the heat battery safe and environmentally friendly?

We have deliberately chosen potassium carbonate as a base material through a selection process that considered, among other aspects, the safety, toxicity, availability and environmental footprint of the material. Potassium carbonate, which is the base material, is a salt that is commonly used within the food industry and as additive to fertilizers. It is inherently safe and has no negative impact on the environment. Furthermore, the production of K2CO3 itself involves binding a significant amount of CO2 and transferring it into the form of the carbonate ion (CO32-).

What polymer system is used or planned to be used to stabilize the composite?

The answer to this question is subject to IP protection and confidentiality at the moment. Making a stable composite is one of the key challenges from the materials perspective.

How much material is needed to operate the battery?

For the applications and the demonstration sites in HEAT-INSYDE we are presently working with amounts of materials ranging between 200 litres and a cubic meter. This is also dependent on the type of application. Generally, we think that the most promising use cases of the standalone heat batteries are more in collective heating solutions, where different houses use the same device. In that respect, the size of the battery and the amount of salt typically depend on the heating amount, which in collective solutions will be larger. Hence, the amount of material needed is highly dependent on the energy demand to be covered.

What is the full ecological footprint of the battery?

A full analysis is one of the ongoing tasks of the project itself and will be presented at the end of the project. However, the base material K2CO3 is fully recyclable with simple means. Furthermore, every C in K2CO3 originates from one molecule of CO2. So, the production involves CO2 capture. Moreover, the components are made from simple metals and are made modular allowing easy recycling. Further optimization of the component materials with respect to recyclability is foreseen.

What temperature is required to charge the battery?

The upper temperature cannot exceed 150 °C as the salt composite will melt. Within the HEAT-INSYDE context and in the present application we use 70 °C, but with Cellcius we are developing a way to charge the battery at 45-40°C.

At which temperature does the battery release its heat?

The battery uses a chemical reaction with water vapor. The temperature at which the heat can be released depends on the temperature of the water that is available. The battery will be able to bring a temperature increase of 40-50 °C. So the battery can deliver heat at 50-60 °C if the source is 10 °C, but also 100 °C if the cold source is 50-60 °C. Tap water supply (typically at 65 °C) and heating (typically in a range of 30-45°C) are therefore excellent application cases for the heat battery.

It is important to note that the performance during discharging is not affected by the charging temperature. In addition, a key difference compared to most common storage applications like water buffers or PCM, is that the energy density of our battery is not different with a high or low charging/discharging temperature.

How much energy is lost in evaporating the water down to extract it?

This question can be approached from two ways. Considering it is a fully closed system also in terms of energy content, per mole of water 40 kJ/mol would need to be invested to gain per mole of water into salt 60 kJ/mol. But in practice, what we do is try to use free available thermal energy from the environment to evaporate the water, meaning that the net energy output is still the amount you get from the salt: 60 kJ/mol.

In previous publications, a semi-open system was presented. Did you change the technology and are you now using a system in high vacuum?

No, we’re not working on a different technology. We are still in the same system and we’re definitely not working under high vacuum. We are working in the pressure range of -0.5 bar and +0.5 bar atmospheric pressure.

What is the storage volume in the heat battery?

The volume of storage required to store a certain amount of energy is determined by the storage capacity. The HEAT-INSYDE battery works at a storage capacity of 600 MJ/m3. This means that for a typical 4-person household (energy demand of 50 MJ/day) and a storage period of 2 weeks the storage volume is comparable to that of a typical refrigerator.

What level of power output can the battery generate?

If we look at the materials, the material can have a power in the order of 50 – 60 W/kg. For the 200 MJ battery we use about 400 kilograms of active material. By multiplying these numbers, you can get quite a high power output of 24 kW. On the other end we have a heat exchanging unit and we are working only with a small amount of active material during one charge or discharge of the system, so currently we are in the order of 2 kW maximum power output. In case in the future we would like to go to higher power output, this is definitely possible, but requires a different design of the battery.

Will it be possible to use this heat battery for both daily and seasonal storage?

Yes technically it is, in fact as the battery can store loss free this is one of the few options to store heat over the season with a relatively small storage volume. In the upcoming Q&A sessions we will discuss in more details why you would or would not like to use the battery as daily or seasonal storage.

How heavy is the device?

The actual storage material has an energy content of 0.5 MJ/kg. The total weight of the heat battery will thus depend on the amount of energy to store. Assuming a storage period of up to 1 weeks and a typical 4-person household (energy demand of 20-50 MJ/day),200 MJ will need to be stored. The storage material will, therefore, roughly weigh 450 kg. However, on top of this another 350 kg need to be added to account for the other heat battery components, such as the exchanger, pumps, etc.

Is the battery a silent product?

Compared to a heat pump, the heat battery is silent. There are moving parts in it, like the ventilator, but because it is a closed loop the noise level is relatively low.

Are you still looking for demo locations?

In the next couple of years, the battery will be further tested in the lab, but also with end-users in the Netherlands, Poland and France. Initially, testing will be conducted with a small group of end-users. We will expand this test group, but most probably not within the H2020 project HEAT-INSYDE.

Is my house suitable for the HEAT-INSYDE battery?

As with most renewable energy innovations, it is desirable for your house to be sufficiently insulated so that low temperature heating can be used. If this is the case, the battery will function excellently in combination with other renewable sources and the necessary storage space is typically in the order of a refrigerator. The size will depend strongly on the type of home (detached, terraced house, apartment), the type of residents (single, family, seniors) and on the use case strategy (e.g., to optimize the local generation of energy for self-consumption) Within the Horizon 2020 project HEAT-INSYDE we will determine the improved performance of the energy systems with the heat battery.

Can the battery be outside on a terrace next to a house?

Definitely, it can be placed outside.

When will the battery become available to the consumer?

The heat battery is not yet commercially available. We are working hard to have a user-ready-prototype available soon. This will take at least another 2 years. The user-ready prototype will need to be tested by end-users and depending on the results, the product will be available for the consumer. In terms of translating the technology into a product and bringing it to the market, we expect to see the first real market applications from 2025 onwards.

The boundary conditions of the HEAT-INSYDE project, within the framework of Horizon 2020, include delivering a heat battery for residential applications. However, we are seeing that there are several other application opportunities, such as in utility buildings, greenhouses and off-grid locations. To bring the technology to the market we introduced a new business vehicle, Cellcius B.V., as a new partner in the consortium. Cellcius is in place to bring this technology to the market but will also explore a variety of applications in different market segments from the one defined for HEAT-INSYDE.

Is it too early to have an idea of what price to expect for a GJ or kWh?

Yes, it’s too early, but we already have a clue about the costs price range which depends on the application, on the configuration, on the size. For typical household application, we expect the costs to be in the order of magnitude of  €10 per MJ. Typically it will amount to a couple of thousand euros per individual house.

What is the return on investment?

This strongly depends on the size, current system and type of house, which makes it hard to give a straightforward answer at this point.